The present invention relates to a stamping method and a stamping apparatus for forming a processed product having a desired shape from a workpiece.
A technique related to stamping is disclosed in Japanese Laid-Open Patent Publication No. 2007-38292. In this conventional technique, a workpiece 100 is processed by the procedure illustrated in FIGS. 17 to 20. First, as illustrated in FIGS. 17 and 18, an upper die base 200 is lowered to press an ejector 121, a stripper 122, and a punch 123 against the top surface of a workpiece 100. A lower die base 300 includes an outer die 112 and an inner die 111. The height of the inner die 111 is small compared to the height of the outer die 112 such that a step 114 is formed between the top surface of the die 111 and the top surface of the die 112.
With reference to FIGS. 19 and 20, the upper die base 200 is continuously lowered together with an ejector 113 of a lower die base 300, which is urged upward by springs 115. In this manner, a processed product portion 110 is formed in the workpiece 100. In this state, the processed product portion 110 includes a flange 102, a body portion 103, and an inner joint portion 108 between the flange 102 and the body portion 103. The processed product portion 110 is connected to the flange 102 through an outer joint portion 109. The processed product portion 110 includes the flange 102 corresponding to an outer periphery and the body portion 103 corresponding to a central portion, which are formed with the joint portion 108 located between the flange 102 and the body portion 103. A recess 104 is formed in the body portion 103.
Then, after the upper die base 200 is raised to release the workpiece 100 from shaping pressure, the outer joint portion 109 is sheared to separate the processed product portion 110 from the workpiece 100, thus completing a processed product, in non-illustrated subsequent steps.
As has been described, in the technique described in Japanese Laid-Open Patent Publication No. 2007-38292, an outer peripheral portion of the processed product portion 110 is sheared at a position between the upper die base 200 and the lower die base 300, which are selectively lifted and lowered at a predetermined stroke, and the recess 104 is formed at an inner side of the processed product portion 110. Accordingly, in the stamping-completed state illustrated in FIG. 20, the thickness of the joint portion 108 between the recess 104 and the flange 102 needs to be great compared to the thickness of the outer joint portion 109, which will be sheared. To satisfy such need, the height of the inner die 111 is set small compared to the height of the outer die 112 to ensure a step 114 between the dies 111, 112 of the lower die base 300. As illustrated in FIGS. 18 and 19, the step 114 causes delay in stamping of a portion corresponding to the joint portion 108, which is close to the recess 104, compared to stamping of a portion corresponding to the joint portion 109, which will be sheared. As a result, the thickness of the inner joint portion 108 becomes great compared to the thickness of the outer joint portion 109.
However, the step 114 forms a gap between the workpiece 100 and the inner die 111 at an early stage of stamping, as illustrated in FIG. 18. This allows deformation of the workpiece 100 inside the gap. Accordingly, the workpiece 100 may deform and cause the processed product portion 110, or, particularly, the body portion 103, to be processed in a bent state. Such deformation may remain in the processed product after stamping or the processed product may recover from the deformation after the stamping. In these cases, errors may be caused in the shape and the size of the processed product compared to the desired shape or the desired size. This decreases accuracy of the processed product.